The preponderance of hydraulic pumps made today fall into one of three broad design types, namely, gear, vane and piston. Piston pumps are further broken down into two design types, namely, valve plate and check ball pumps. As examples, the pumps depicted in U.S. Pat. Nos. 4,579,043 (Nikolaus et al.) and 4,602,554 (Wagenseil et al.) are of the valve plate type while that depicted in U.S. Pat. No. 3,514,223 (Hare) is a check ball pump. Some features of valve plate and check ball pumps will now be described.
Valve plate pumps include a cylinder barrel having a number of pistons reciprocating in it. Such barrel is coupled to the pump shaft and rotates with the shaft and as a consequence, the pistons in a valve plate pump both rotate with the pump barrel and reciprocate in such barrel.
Such pistons are caused to reciprocate by rotating the barrel with respect to a stationary "swash plate" or wobble plate. Barrel rotation urges pistons toward a fluid-porting cover as the piston shoe moves along the "rising" part of the wobble plate. Fluid, e.g., hydraulic oil, between the distal end of the piston and the cover is expelled through the cover and into a tube or hose to perform useful work. As the pistons move along the "alling" part of the wobble plate, they move away from the cover and draw fluid into the enlarging cavity between the cover and the piston distal end.
At their proximal ends, the pistons typically have a flat-faced shoe that rides along the angled face of the wobble plate. During pump operation, it is important to maintain the shoe in contact with such face--shoe "liftoff" can result in a damaged shoe and, in a more aggravated case, in a pump that destroys itself.
In a valve plate piston pump, there are a number of ways to hold the piston shoes in contact with the wobble plate. One way is to use an annular plate having a number of holes formed therein equal to the number of pistons. Such plate, shown in the Wagenseil et al. patent, for example, and identified therein as a "contact pressure plate," closely resembles the dial plate of a rotary-dial-type telephone. Other ways to hold a piston in contact with an undulating surface in a valve plate pump is by an internal spring (U.S. Pat. No. 5,320,498 (Fuchida) or by a "head-grasping" arrangement as shown in U.S. Pat. No. 4,860,641 (Spears).
In a typical check ball pump like that shown in the Hare patent, the barrel having the reciprocating pistons does not rotate. On the other hand, the wobble plate (or a thrust plate analogous to the wobble plate) rotates when driven by the pump shaft. The interior of the pump housing is flooded with oil and as each piston moves away from the front cover, the cavity between the piston distal end and the pump cover fills with oil. Filling is through one or more piston "fill holes" in fluid communication with the flooded housing interior and the piston cavity and oil which flows through such holes then flows across an inlet check valve inside the piston. This part of piston travel is often referred to as the "suction stroke."
As the wobble plate continues rotation and a piston moves toward the front cover, its discharge check valve (mounted in the pump cover) opens and the volume of oil in the aforedescribed cavity is expelled through the cover and into a tube or hose to perform useful work. This part of piston travel is often referred to as the "discharge stroke" or "pressure stroke."
While check valve pumps have been available for decades and have proven sturdy and reliable even in harsh operating environments, it has become apparent that steps needed to be taken to obtain greater displacement from a given frame size. However, certain structural features, seemingly inherent in pumps of this type, militate against significant increases in such displacement.
Such features relate to the need to hold the shoe of each reciprocating piston in intimate contact with the rotating wobble plate. A common technique, depicted in the Hare patent noted above, involves a spring retainer plate attached at a reduced-diameter "neck" between the spherical piston head and the cylindrical body. A trepan groove is formed in the pump barrel concentric with each piston bore and a compression-type piston return spring is mounted in the groove. When the piston is inserted in the barrel, the spring bears against the retainer plate and urges the piston toward the wobble plate.
A fact of this arrangement is that for a given housing cavity size (and pump size and "mass"), the spring and retainer plate occupy a significant part of the cavity volume. Another fact is that as the springs and retainer plates move through the oil contained in the cavity, there is necessarily some loss in efficiency. Simply put, the oil resists movement of the plate and spring.
Yet another aspect of the above-noted check valve pump design is that the piston fill holes are required to be relatively small. As a consequence, inlet supercharge is indicated for many installations since the pistons would not other-wise fill properly. Users often resist having to provide such supercharge and the maximum speed of the pump is somewhat limited. Yet another fact is that if supercharge is increased beyond a few pounds per square inch, one has to consider the use of high pressure shaft seals.
The known arrangement (as typified by the pump of the Hare patent) involves a relatively large number of parts. Further, many of such parts were required to be machined in a way that, in view of the invention, is unnecessary. For example, the pistons of the pump shown in the Hare patent have "necked-down" portions machined therein adjacent to the piston shoe. A snap ring groove is machined in such necked-down portion to receive a snap ring for holding the shoe on the end of the piston. And because of the relatively large number of parts, the time required to assemble a pump of the type shown in such patent is rather significant.
An improved piston pump which overcomes some of the problems and shortcomings of known pumps would be an important advance in the art.